Patients with hypertrophic subaortic stenosis usually have supernormal left ventricular function at rest and normal function during exercise despite the presence of symptoms requiring surgical intervention. We used radionuclide cineangiography at rest and during exercise to assess the effects of septal myotomy and myectomy, which involves the removal of moderate amounts of myocardium on left ventricular function. We found the operation usually does not importantly impair left ventricular function, at rest or during exercise stress.